On s-process nucleosynthesis in thermally pulsing stars

The detailed operation of s-process nucleosynthesis in the convective shell of a thermally pulsating star is investigated for the case where the Ne-22 (alpha, n) Mg-25 reaction is adopted as a neutron source. A nuclear reaction network extending from Ne-22 to Bi-209 is constructed for tracing the thermonuclear evolution of the matter in the convective shell; the abundances emerging from a 'standard model' that incorporates the relevant properties of detailed models of pulsating stars are found to approach those obtained in the asymptotic limit of a previous approximate calculation. The time variations of the abundance distributions are described, and the number of pulse epochs required for the realization of the asymptotic limiting solutions is determined as a function of the degree of overlap between successive convective shells. Neutron competition from nuclei lighter than iron is discussed together with the associated nucleosynthesis of neutron-rich isotopes in the same atomic-mass range. Analytical expressions exhibiting the controlling features of thermal-pulse neutron exposures are derived, implications of the results for galactic nucleosynthesis are considered, and the s-process distributions estimated for various stellar models are interpreted.